airdrop issue between iMac (late 2009) & iPad air 2

Apologies if posted in the wrong forum but not sure where to post this query

I have an iMac (late 2009) using OS X 10.10.5 & an iPad air 2

How can I transfer a file from the iPad to the iMac?

I've switched on airdrop with access to everyone on the ipad

I can open an airdrop window on the imac

However, neither of the devices are showing up in the other one's window

What have I done wrong?